The basic components of a gas chromatograph include an injection port for introducing a sample of matter to be examined into a stream of carrier medium, a column attached to the injection port and constructed in the form of a helix of tubing containing chemicals that cause some of the constituents of the sample to elute at different times, a temperature-controlled zone in which the column is mounted, and a detector for producing a signal indicative of the concentration of the constituents being eluted. An integrator may be employed for integrating the signal so as to provide information as to the quantity of each constituent.
In the typical gas chromatograph, the temperature-controlled zone is constructed as an oven. The injection port and detector are attached to respective pneumatic fittings on the oven, and the separation column is attached between the pneumatic fittings and located within the oven. The oven typically comprises a thermally insulated oven housing having a door to permit access to the oven interior, a controlled heating element, and a motor-driven stirring fan. The stirring fan continuously mixes the air contained within the oven housing so as to minimize temperature gradients therein that could adversely affect the performance of the chemical process occurring within the column. In analyzing most samples, the heating element is controlled so as to increase the temperature of the oven from a minimum initial value to a maximum final value. Before introduction of the next sample into the column, the temperature of the oven is returned to its initial value. Whereas rapid cooling can be effected by simply opening the door, this heats up the front surfaces of the oven to an objectionable degree. Accordingly, it is known to operate the stirring fan to draw cool ambient air into the oven through an intake port and to expel hot air from the oven through an exhaust port in such manner as to avoid heating the outer surfaces with which the operator is likely to come in contact.
Some gas chromatographs employ combustible gasses such as hydrogen as the carder gas or detector gas. Even though the injection port, detector, separation column, and pneumatic fittings in the typical chromatograph are designed to minimize leakage of such a combustible gas into the oven interior, one may nonetheless consider a pneumatic fault mode wherein a gas leak could occur and sufficient gas could accumulate in the oven so as to pose an unsafe condition.